Ryan Shea| Dec 12, 2011
How do I improve boiler efficiency?
Instrumentation is essential. Why?
While it is possible to operate and control a boiler with indicators and recorders alone, efficient and reliable operation through manual control is not optimal these days. Early boilers were smaller and operated manually, but rising demand for, and cost of, steam production has resulted in larger boilers with increased output. More careful boiler control calls for process instrumentation.
Manual air control is inaccurate and unreliable, particularly when flow rates drop below 25% of maximum load. Traditional control methods set the fuel-air ratio via a pressure controller connected to mechanical linkages between the fuel control valve and the airflow damper. This solution cannot correctly compensate for process variables and is slow to respond to varying load conditions. Air dampers are not accurate due to factors such as non-linearity, leakage, and hysteresis, as is the use of variable speed or variable pitch fans. Neither of these control elements can be solely depended on for accurate control.
It’s all about the air.
Airflow is one of the most important components of boiler control and is critical for safe, reliable, and efficient operation. Incorrect airflow can result in toxic gas production, flame outs, fuel-rich conditions and explosion, inefficient operation, erratic cycling, and other issues. Boiler operators need continuous information to adjust the firing rates and distribution of fuel and air to the burners. This can sometimes be difficult to obtain due to non-linear fan and damper characteristics, air density differences, and other factors.
It’s common to feed excess air to the boiler at all loads to ensure complete combustion of the fuel. However, this practice can reduce efficiency. Too much air means excess fan power and stack loss from unused oxygen. A careful balance between fuel and air is necessary for optimum efficiency, which can only be obtained through careful, multivariable measurement of airflow, fuel flow, and analytical analysis of the stack gases.
How do I know how much air I really have?
Traditionally, duct pressure has been the way to monitor air fed to the boiler, but soot and leakage can affect the accuracy of your measurement, which hampers your ability to control your fuel-to-air ratio. A better way to optimize airflow is by using accurate flow devices to provide feedback to a control system that can automatically adjust the damper positions and fan speeds.
How do I make the most of it?
The goal of airflow optimization is to minimize thermal losses and prevention of a starved boiler. Minimizing excess air reduces stack temperature and thermal loss and can only be done utilizing high accuracy airflow measurement devices. The fuel saving potential in a boiler can exceed 20% in some cases and can lead to substantial savings.
The best approach is to utilize a suitable meter run, venturi, or duct sensor that will ensure all air enters the fan. Averaging Pitot tubes, venturi tubes, and orifice plate primary elements utilizing high accuracy DP transmitters (click here for a closer look) are the best suited and most preferred flow measurement devices. Mass compensation for density variations is also preferred and can be calculated utilizing multivariable transmitters, external flow computers, or control systems that take in separate flow, pressure, and temperature signals. The control system can then employ a suitable control strategy and throttle the dampers and fans to provide the correct airflow to the burners.
How are you optimizing your boiler efficiency?
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